Conventionally known is a technique for detecting an electric current through a conductive path with an electric current sensor. As this type of electric current sensor, an electric current sensor provided with a Hall element needs to be used in accordance with the magnitude of an electric current, and thus if the electric current is too large, a large electric current sensor needs to be used, which does not meet the demand for a decrease in its size and is problematic in that its manufacturing cost will increase. In view of this, a technique is known in which a conductive path is provided with a parallel circuitry, an electric current is shunted, connecting resistance elements to this parallel circuitry, and the shunt current is detected. In JP 2004-294306A, a conductive line for connecting a driver and a motor branches to a main conductive line and a bypass line, a shunt resistor is connected to the main conductive line, and a voltage division resistor and a non-contact electric current sensor are connected in series to the bypass line. Doing so makes electric current flow through the lines based on the ratio of the shunt resistor, the voltage division resistor, and the internal input resistance of the non-contact electric current sensor, and thus it is possible to reduce the size of the electric current sensor compared to the case in which the overall flowing current is detected with the electric current sensor.
Incidentally, with the configuration of JP 2004-294306A, in order to detect the electric current, the shunt resistor and the voltage division resistor are connected to a parallel circuitry, and thus power loss occurs due to the electric current flowing through these resistors, which is problematic in that the advantage that the non-contact electric current sensor does not lose power significantly was not obtained sufficiently.
The present invention was made based on the above-described circumstances, and an object of the present invention is to reduce loss caused by resistors and detect the electric current of a conductive path.